Thin-film circuit device, method for manufacturing thin-film circuit device, and electronic apparatus

ABSTRACT

A thin-film circuit device includes a substrate and a thin-film circuit layer, disposed on the substrate, having an element region and a low-strength region. The element region includes thin-film elements. The low-strength region extends between an end portion of the thin-film circuit layer and the element region and has a mechanical strength less than that of the surroundings of the low-strength region.

BACKGROUND

1. Technical Field

The present invention relates to a thin-film circuit device (thin-filmsemiconductor device) including a thin-film circuit layer, disposed on aflexible substrate, including semiconductor elements; a method formanufacturing such a thin-film circuit device; and an electronicapparatus including such a thin-film circuit device.

2. Related Art

Thin-film circuit devices include substrates; thin-film circuit layers,disposed on the substrates, including semiconductor elements; and othercomponents. The substrates are usually prepared from single-crystalsilicon wafers, quartz glass wafers, heat-resistant glass wafers, resinfilms, or the like. These wafers are selected depending on the requiredperformance and functions of the thin-film circuit devices. Inparticular, the resin films are preferable because a substrate preparedfrom a resin film is thin and flexible and therefore a thin-film circuitdevice including the substrate is light-weighted and flexible.

Examples of methods for manufacturing thin-film circuit devicesincluding substrates prepared from resin films include a method in whicha semiconductor layer, an insulating layer, a metal layer, and the likeare deposited on a resin film in that order and a method in which athin-film circuit layer is formed on a heat-resistant substrate,released from the heat-resistant substrate, and then joined to a resinfilm (a release transfer process). Japanese Unexamined PatentApplication Publication No. 10-125931 (hereinafter referred to as PatentDocument 1) discloses a method for manufacturing a thin-film circuitdevice by a release transfer process.

A thin-film circuit device including a substrate prepared from a resinfilm can have defects due to a difference in physical property betweenthe substrate and a thin-film circuit layer.

The thin-film circuit layer usually includes an inorganic thin-filmformed by depositing an inorganic material on the substrate by achemical vapor deposition (CVD) process or a sputtering process. Theinorganic thin-film has a large elastic constant of several tengigapascals and a small linear expansion coefficient of several to 10 ormore ppm/K. On the other hand, the resin film has a large linearexpansion coefficient of about 10 to 50 ppm/K.

In the thin-film circuit device, since such materials having differentphysical properties are joined to each other, a change in temperaturecauses thermal stresses in the substrate and the thin-film circuit layerbecause of the difference in linear expansion coefficient. Since thethin-film circuit layer usually has a small thickness of severalmicrometers and a small cross-sectional area, a large thermal stress isdeveloped in the thin-film circuit layer. If the thermal stress exceedsthe breaking strength of the thin-film circuit layer, the thin-filmcircuit layer is broken. This causes the failure of the thin-filmcircuit device.

When the substrate is distorted or bent, a bending stress is developedin the thin-film circuit layer. Even a small strain causes a largestress in the thin-film circuit layer because the thin-film circuitlayer has a large elastic constant of several ten gigapascals.Therefore, the stress caused by distortion such as bending can break thethin-film circuit layer.

End portions of the thin-film circuit device can have fine cracks ornotches formed in a cutting step in which the thin-film circuit deviceis separated from other thin-film circuit devices. Large stresses aredeveloped in the end portions thereof because of stress concentration;hence, the end portions thereof serve as weak points that may cause thebreakage of the thin-film circuit device.

SUMMARY

An advantage of an aspect of the invention is to provide a thin-filmcircuit device having high reliability.

An advantage of another aspect of the invention is to provide a methodfor manufacturing such a thin-film circuit device. An advantage ofanother aspect of the invention is to provide an electronic apparatusincluding such a thin-film circuit device.

A first aspect of the present invention provides a thin-film circuitdevice, which includes a substrate and a thin-film circuit layer,disposed on the substrate, having an element region and a low-strengthregion. The element region includes thin-film elements. The low-strengthregion extends between an end portion of the thin-film circuit layer andthe element region and has a mechanical strength less than that of thesurroundings of the low-strength region.

A second aspect of the present invention provides a thin-film circuitdevice, which includes a substrate and a thin-film circuit layer,disposed on the substrate, having an element region and a low-strengthregion. The element region includes thin-film elements. The low-strengthregion extends between an end portion of the thin-film circuit layer andthe element region to surround the element region and has a mechanicalstrength less than that of the surroundings of the low-strength region.

Since the thin-film circuit devices have the above configurations, thelow-strength regions can prevent cracks developed in end portions of thethin-film circuit devices from reaching the element regions. In thethin-film circuit device according to the second aspect, thelow-strength region surrounds the element region, cracks can beprevented from propagating to the element region from outside.

In the thin-film circuit device according to the first or second aspect,the low-strength region preferably surrounds the element region severaltimes. This securely prevents cracks from propagating to the elementregion.

In the thin-film circuit device according to the first or second aspect,the low-strength region preferably has a groove surrounding the elementregion. This allows the low-strength region to partly have a thicknessless than that of other regions. Therefore, the low-strength region hasa mechanical strength less than that of the surroundings thereof.

In the thin-film circuit device according to the first or second aspect,the low-strength region preferably has a plurality of groovessurrounding the element region. This securely prevents cracks frompropagating to the element region.

In the thin-film circuit device according to the first or second aspect,the groove or grooves preferably have a V shape, an inverted trapezoidalshape, a rectangular shape, a semi-circular shape, or a semi-ellipticalshape in cross section or partly have any one of these shapes in crosssection. The groove or grooves preferably have a depth that is 50% ormore of the thickness of the thin-film circuit layer and more preferablyabout 50% to 85% thereof.

When the groove or grooves have a V shape in cross section, thelow-strength region has a linear portion which is located at the bottomof the groove and which has the smallest thickness or has linearportions which are located at the bottoms of the respective grooves andwhich have the smallest thickness. This allows the low-strength regionto function well.

When the groove or grooves have a semi-elliptical shape or partly have asemi-elliptical shape in cross section, the low-strength region has alinear portion which is located at the bottom of the groove and whichhas the smallest thickness or has linear portions which are located atthe bottoms of the respective grooves and which have the smallestthickness. This allows the low-strength region to function well.

In the thin-film circuit device according to the first or second aspect,when the low-strength region has the grooves, the element region can besecurely protected. This allows the thin-film circuit device to havehigh reliability.

A third aspect of the present invention provides an electronic apparatusincluding the thin-film circuit device of the first or second aspect.The thin-film circuit device hardly breaks down; hence, the electronicapparatus has high reliability.

A fourth aspect of the present invention provides a method formanufacturing a thin-film circuit device including a substrate and athin-film circuit layer disposed on the substrate. The method includesforming an element region including thin-film elements on the substrateand forming a low-strength region having a low mechanical strengtharound the element region. The low-strength region is formedsimultaneously with the formation of contact holes in the elementregion.

The method is effective in manufacturing the thin-film circuit device atlow cost.

In the method, the low-strength region is preferably formed by etchingthe thin-film circuit layer. This allows the contact holes and thelow-strength region to be formed in one step, resulting in a reductionin manufacturing cost.

The method preferably further includes fabricating the thin-film circuitlayer on a heat-resistant substrate, releasing the thin-film circuitlayer from the heat-resistant substrate, and then transferring thethin-film circuit layer to a flexible substrate. This technique iscalled a release transfer process and is useful in providing a thin-filmcircuit layer, including high-performance thin-film transistorsfabricated at high temperature, on a resin substrate having low heatresistance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with reference to theaccompanying drawings, wherein like numbers reference like elements.

FIG. 1 is a perspective view of a thin-film circuit device according toa first embodiment of the present invention.

FIG. 2 is an illustration showing a step included in a method formanufacturing the thin-film circuit device of the first embodiment.

FIG. 3A is a plan view of the thin-film circuit device of the firstembodiment, FIG. 3B is an illustration of an end portion X of anotherthin-film circuit device that has no low-strength region, and FIG. 3C isan illustration of an end portion X of the thin-film circuit device ofthe first embodiment, this end portion X having a portion of alow-strength region.

FIG. 4A is a plan view of the thin-film circuit device of the firstembodiment and FIGS. 4B, 4C, and 4D are enlarged sectional views of thethin-film circuit device of the first embodiment taken along the lineA-A′.

FIG. 5A is a plan view of a thin-film circuit device according to asecond embodiment of the present invention and FIGS. 5B to 5C areenlarged sectional views of this thin-film circuit device taken alongthe line B-B′.

FIG. 6A is a plan view of the thin-film circuit device of the secondembodiment and FIGS. 6B(a) to 6B(e) are illustrations showing variationsof the pattern of grooves present in this thin-film circuit device.

FIGS. 7A to 7G are illustrations showing steps included in a method formanufacturing a thin-film circuit device according to a third embodimentof the present invention.

FIGS. 8A to 8G are illustrations showing steps included in a method formanufacturing a thin-film circuit device according to a fourthembodiment of the present invention.

FIGS. 9A to 9G are illustrations showing steps included in a method formanufacturing a thin-film circuit device according to a sixth embodimentof the present invention.

FIGS. 10A to 10E are illustrations showing steps included in a methodfor manufacturing a thin-film circuit device according to a seventhembodiment of the present invention.

FIGS. 11A and 11B are illustrations showing steps included in a methodfor manufacturing a thin-film circuit device according to an eighthembodiment of the present invention.

FIGS. 12A and 12B are illustrations showing steps included in a methodfor manufacturing a thin-film circuit device according to a ninthembodiment of the present invention.

FIGS. 13A to 13D are illustrations showing steps included in a methodfor manufacturing a thin-film circuit device according to a tenthembodiment of the present invention.

FIGS. 14A to 14D are illustrations showing steps included in a methodfor manufacturing a thin-film circuit device according to a eleventhembodiment of the present invention.

FIGS. 15A to 15C are illustrations of examples of an electronicapparatus including the thin-film circuit device according to any one ofthe above embodiments.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the present invention will now be described withreference to accompanying drawings.

Thin-film circuit devices according to preferred embodiments of thepresent invention include substrates and thin-film circuit layersdisposed thereon. Low-strength regions having a low mechanical strengthextend between end portions of the thin-film circuit devices and elementregions for forming thin-film elements to surround the element regions.

Fine cracks and/or irregularities are usually formed in the end portionsof the thin-film circuit devices in a cutting step in which thethin-film circuit devices are separated from other thin-film circuitdevices. It is very difficult for any technique or process to completelyprevent the formation of such fine cracks and/or irregularities.According to the preferred embodiments below, although cracks aredeveloped in the substrates or the end portions of the thin-film circuitdevices and propagate inward, the cracks are guided into thelow-strength regions and prevented from further propagating inward,thereby preventing the breakage of the element regions.

First Embodiment

A thin-film circuit device according to a first embodiment of thepresent invention will now be described with reference to FIGS. 1 and 2.

FIG. 1 is a perspective view of the thin-film circuit device. Withreference to FIG. 1, the thin-film circuit device is represented byreference numeral 100 and includes a substrate 101 and a thin-filmcircuit layer 102 disposed thereon. In this embodiment, the thin-filmcircuit layer 102 has the same size as that of the substrate 101 and isformed on the substrate 101 so as to have an extremely small thickness.The thin-film circuit layer 102 may have a size different from that ofthe substrate 101.

The thin-film circuit layer 102 may be directly formed on the substrate101 or may be formed on another substrate and then joined to thesubstrate 101 by the above release transfer process using an adhesive.

The thin-film circuit layer 102 includes an element region 103 and alow-strength region 104 surrounding the element region 103. The elementregion 103 has a predetermined function and includes, for example, anelectric circuit, a display circuit, a mechanical microstructure, andthe like. The low-strength region 104 has a mechanical strength lessthan that of other regions included in the thin-film circuit layer 102.The mechanical strength of the low-strength region 104 can be adjustedin such a manner that the low-strength region 104 is formed so as tohave a small thickness, formed from a material having low mechanicalstrength, or subjected to laser irradiation so as to have low mechanicalstrength as described below. The low-strength region 104 extends betweenthe element region 103 and an end portion of the substrate 101 (or anend portion of the thin-film circuit layer 102) to surround the elementregion 103. The low-strength region 104 and an additional low-strengthregion may surround the element region 103 as described below. Thelow-strength region 104 need not completely surround the element region103 but may partially surround the element region 103. This issufficient to achieve the effect of preventing cracks from propagatinginward from the low-strength region 104.

FIG. 2 is an illustration showing a step included in a method formanufacturing the thin-film circuit device 100. As shown in FIG. 2, athin-film circuit section 12 is formed on a wafer 11 with a large size.The thin-film circuit section 12 includes the element region 103, otherelement regions, the low-strength region 104, and other low-strengthregions. The thin-film circuit section 12 has, for example, the samesize as that of the wafer 11 and is formed on the wafer 11 so as to havean extremely small thickness.

The wafer 11 is cut into chips, which each have one of the elementregion 103 and the element regions and one of the low-strength region104 and the low-strength regions. Examples of a process for cutting thewafer 11 include a mechanical cutting process using a cutting whetstone,a knife, or a pair of scissors; a laser scribing process; and anotherprocess. The thin-film circuit device 100 prepared by processing one ofthe chips can be directly used as a product or used as a component.

A large number of the element and low-strength regions are formed on thewafer 11 and the resulting wafer 11 is cut into the chips as describedabove. Therefore, the thin-film circuit device 100 can be preparedefficiently.

A function of the low-strength region 104 will now be described withreference to FIGS. 3A to 3C.

FIG. 3A is a plan view of the thin-film circuit device 100. FIG. 3B isan illustration of an end portion X of another thin-film circuit devicethat has no low-strength region. FIG. 3C is an illustration of an endportion X of the thin-film circuit device 100 that has a portion of thelow-strength region 104.

The end portion X of the thin-film circuit device 100 has small notchesY formed in the cutting step. The notches Y can be present only in oneof the substrate 101 and the thin-film circuit layer 102 or present inboth of the substrate 101 and the thin-film circuit layer 102. The sizeand number of the notches Y depend on properties of at least one of thesubstrate 101 and the thin-film circuit layer 102 and a process forcutting the wafer 11. However, it is very difficult for any technique orprocess to completely prevent the formation of the notches Y.

The distortion or bend of the thin-film circuit device 100 or the changein the temperature of the thin-film circuit device 100 causes mechanicalor thermal stresses in the substrate 101 and the thin-film circuit layer102. Such stresses are concentrated on the edges of the notches Ypresent in the end portion X of the thin-film circuit device 100,thereby causing large cracks Z. Since the-end portion X of the thin-filmcircuit device has no low-strength region as shown in FIG. 3B, suchcracks Z propagate to inner portions of a thin-film circuit layer toreach an element region included in this thin-film circuit device. Thiscan lead to a failure of this thin-film circuit device.

As shown in FIG. 3C, the low-strength region 104, which extends betweenthe element region 103 and an end portion of the substrate 101, preventsthe cracks Z present in the end portion X of the thin-film circuitdevice 100 from reaching the element region 103. When the cracks Zpropagate toward an inner portion of the thin-film circuit layer 102,the cracks Z reach the low-strength region 104. The low-strength region104 has a smaller mechanical strength as compared to the surroundingsthereof. After the cracks Z reach the low-strength region 104, thecracks Z propagate in the low-strength region 104 in the direction inwhich the low-strength region 104 extends, because the cracks Zpropagate toward a portion having a smaller mechanical strength. Oncethe cracks Z propagate in the low-strength region 104, an outer portionof the thin-film circuit layer 102 is separated from an inner portion ofthe thin-film circuit layer 102 with the cracks Z. Therefore, even ifother cracks propagate to the low-strength region 104 from other endportions, these cracks can be prevented from propagating out of thelow-strength region 104.

As described above, the low-strength region 104 extends between theelement region 103 and the end portion of the substrate 101 to surroundthe element region 103. This prevents the cracks Z, which propagate fromthe end portion of the substrate 101, from reaching the element region103, thereby preventing a failure of the thin-film circuit device 100.This allows the thin-film circuit device 100 to have high resistance toeither one or both of a mechanical stress and a thermal stress.

The configuration of the low-strength region 104 will now be describedin detail with reference to FIGS. 4A to 4D.

FIG. 4A is a plan view of the thin-film circuit device 100. FIGS. 4B,4C, and 4D are enlarged sectional views of the thin-film circuit device100 taken along the line A-A′.

As shown in FIG. 4B, the low-strength region 104 has a groove extendingin the thin-film circuit layer 102. The groove has an invertedtrapezoidal or rectangular shape in cross section. Therefore, a portionof the low-strength region 104 that corresponds to the bottom of thegroove has a smaller thickness as compared to the surroundings of thelow-strength region 104. This allows the low-strength region 104 to havea smaller mechanical strength as compared to the surroundings thereof.

Alternatively, the groove may have a V shape in cross section as shownin FIG. 4C. This allows the low-strength region 104 to have a smallermechanical strength as compared to the surroundings thereof.

With reference to FIG. 4C, the distance between the element region 103to an end of the thin-film circuit layer 102 is preferably about 3 mm(3,000 μm) or less. Mobile displays including thin-film transistors needto have a small size and therefore the thin-film circuit device 100needs to have a narrow frame; hence, the distance therebetween is morepreferably about 1 mm or less. The inequality Lb≧La/3 is preferablysatisfied, wherein La represents the distance between the element region103 and an end of the thin-film circuit layer 102 and Lb represents thedistance between the element region 103 and the groove. The equationLb=La/2 is more preferably satisfied because the groove is locatedsubstantially equidistantly from the element region 103 and an end ofthe thin-film circuit layer 102.

The groove preferably has a depth equal to 50% or more of the thicknessof the thin-film circuit layer 102 as described below. The groovepreferably reaches a silicon dioxide layer (the lowermost layer forTFTs) located under the thin-film circuit layer 102. This is becausecracks can arise from the lowermost layer for the TFTs (because of theapplication of shear stresses between a film and the TFTs to thelowermost layer) and then readily propagate in the low-strength region104. In consideration of the configuration of the TFTs, if the groove isformed by grinding the thin-film circuit layer 102 so as to extend fromthe upper face of the thin-film circuit layer 102 to a depth equal toabout 85% of the thickness of the thin-film circuit layer 102, thegroove will reach the silicon dioxide layer on a mathematical basis. TheTFTs usually have a thickness of several to ten micrometers. Therefore,the inequality t_(b)≧t_(a)/2 is preferably satisfied, wherein t_(a)represents the thickness of the TFTs and t_(b) represents the depth ofthe groove.

In consideration of a function of the low-strength region 104 subjectedto stress concentration, the width and depth of the groove is preferablysubstantially equal to each other. The width of the groove may begreater than the depth thereof.

The groove can be formed in the low-strength region 104 by an etchingprocess for semiconductor manufacture. Alternatively, the groove can beformed by a sandblast process, a water jet process, a laser process, oranother process. The sandblast process is advantageous in that thegroove can be formed so as to have a small width and the depth of thegroove can be controlled in the order of several micrometers. The waterjet process is advantageous in that both an organic material and aninorganic material can be uniformly processed. The laser processpreferably uses femtosecond laser pulses because the groove can bereadily formed with high efficiency.

Alternatively, the groove may have a semi-elliptical shape in crosssection as shown in FIG. 4D. This also allows the low-strength region104 to have a smaller mechanical strength as compared to thesurroundings thereof.

The groove may have a semi-circular shape in cross section other thanthe V shape, the inverted trapezoidal shape, the rectangular shape, andthe semi-elliptical shape or may partly have any one of these shapes incross section.

Second Embodiment

FIG. 5A shows a thin-film circuit device 100 according to a secondembodiment of the present invention. The thin-film circuit device 100,as well as that of the first embodiment, includes a substrate 101 and athin-film circuit layer 102 including an element region 103 and alow-strength region 104 surrounding the element region 103. Thelow-strength region 104 has a plurality of grooves.

FIGS. 5B to 5C are enlarged sectional views of the thin-film circuitdevice 100, shown in FIG. 5A, taken along the line B-B′.

The grooves may have a rectangular or inverted trapezoidal shape incross section as shown in FIG. 5B, a V shape in cross section as shownin FIG. 5C, or a semi-elliptical shape in cross section as shown in FIG.5D. Alternatively, the grooves may have a semi-circular shape in crosssection or may partly have any one of these shapes in cross section.

According to this configuration, even if cracks are developed in an endportion of the thin-film circuit device 100, the risk that the crackspropagate through the low-strength region 104 to extend inward can bereduced. This allows the thin-film circuit device 100 to have highreliability.

With reference to FIG. 5B to 5D, the number of the grooves is three.However, the number thereof is not limited to three. An increase in thenumber of the grooves enhances the ability of the low-strength region104 to prevent the propagation of the cracks. A reduction in the numberof the grooves reduces the area of the low-strength region 104 as longas the number thereof is sufficient to prevent the propagation of thecracks. This leads to a reduction in the size of the thin-film circuitdevice 100.

Accordingly, the number of the grooves is preferably two to five andmore preferably two or three.

An increase in the depth of the grooves enhances the ability of thelow-strength region 104 to prevent the propagation of the cracks.Therefore, the depth of the grooves is preferably 20% or more of thethickness of the thin-film circuit layer 102 and more preferably 50% ormore of the thickness thereof.

FIG. 6A is a plan view of the thin-film circuit device 100. FIGS. 6B(a)to 6B(e) are illustrations showing variations of the pattern of thegrooves. The grooves are straight as shown in FIG. 6B(a).

Alternatively, the grooves may be as follows: at least one of thegrooves is straight and long (this groove is referred to as a straightgroove) and the others are straight and short and are arranged in astraight line (these grooves are referred to as spaced grooves) as shownin FIG. 6B(b). The straight groove is located close to the elementregion 103 and the spaced grooves are located outside the straightgroove. This allows the thin-film circuit device 100 to have asufficient strength and prevents the cracks from propagating from anouter portion (an end face of a chip).

The spaced grooves may be arranged in two straight lines such that thespaces between the spaced grooves are covered with the correspondingspaced grooves when the spaced grooves are viewed from a side of thesubstrate 101 as shown in FIG. 6B(c). This also allows the thin-filmcircuit device 100 to have a sufficient strength and prevents the cracksfrom propagating from an outer portion (an end face of a chip).

One of the grooves may form periodic triangular waves as shown in FIG.6B(d). This configuration is effective in distributing the cracks orstresses propagating from sides of the substrate 101 in variousdirections. Portions of this groove extend in the propagation directionsof the cracks or stresses; hence, the cracks or the stresses can beeffectively guided to this groove.

One of the grooves may form periodic rectangular waves as shown in FIG.6B(e). This configuration is also effective in distributing the cracksor stresses propagating from sides of the substrate 101 in variousdirections. Portions of this groove extend in the propagation directionsof the cracks or stresses; hence, the cracks or the stresses can beeffectively guided to this groove.

In the grooves shown in FIG. 6B(d) or 6B(e), the corners of thetriangular or rectangular waves may be rounded. The patterns shown inFIGS. 6B(a) to 6B(e) may be used in combination.

The grooves preferably extend uniformly along the four sides of thesubstrate 101. Although the cracks can propagate in any direction, thegrooves extending uniformly along the four sides thereof can securelyprevent the cracks from reaching the element region 103.

Third Embodiment

A method for manufacturing a thin-film circuit device according to athird embodiment of the present invention will now be described withreference to FIGS. 7A to 7G. In the method, a silicon thin-film forforming planar thin-film transistors is used as an etching stopper in astep of forming grooves having a rectangular shape in cross section.

As shown in FIG. 7A, a protective layer 201 is formed on a substrate 101by a CVD process or another process using silicon dioxide or anothercompound. A silicon layer is formed on the protective layer 201 by a CVDprocess or another process. The silicon layer is subjected to lasercrystallization and then heat treatment, whereby the silicon layer isconverted into a polysilicon layer 202.

As shown in FIG. 7B, the polysilicon layer 202 is patterned, whereby atransistor-forming region and a low-strength region 104 serving as anetching stopper region are formed. Silicon dioxide is deposited overthese regions by a CVD process, whereby a gate insulating layer 203 isformed.

As shown in FIG. 7C, a layer of a conductive material such as aluminumis deposited on the gate insulating layer 203 by a sputtering process oranother process and then patterned, whereby a gate electrode-wiringlayer 204 is formed. Impurities are implanted into thetransistor-forming region using the gate electrode-wiring layer 204 as amask. The impurities are then activated by heat treatment or the like,whereby source regions and drain regions are formed.

As shown in FIG. 7D, silicon dioxide is deposited over the gateinsulating layer 203 and the gate electrode-wiring layer 204 by a CVDprocess, whereby an interlayer insulating layer 205 is formed.

As shown in FIG. 7E, the interlayer insulating layer 205 and the gateinsulating layer 203 are patterned by an etching process, wherebycontact holes are formed in the transistor-forming region so as toextend to the respective source and drain regions. In this step, thegrooves are formed in the low-strength region 104. An etching solutionused contains, for example, hydrofluoric acid (HF) and water (H₂O) so asto have a large selective etching ratio with respect to silicon dioxideand silicon. The polysilicon layer 202 serves as an etching stopper.

As shown in FIG. 7F, a layer of a conductive material such as aluminumis deposited on the interlayer insulating layer 205 by a sputteringprocess and then patterned, whereby source and drain electrodes 206 areformed.

As shown in FIG. 7G, silicon nitride or silicon dioxide is depositedover the interlayer insulating layer 205 and the source and drainelectrodes 206, whereby a protective layer 207 is formed. Contact holesare formed in the protective layer 207. A layer of a conductive materialis deposited on the protective layer 207 and then patterned, whereby awiring layer 208 connected to transistor electrodes is formed.

This provides a thin-film circuit layer 102 disposed on the substrate101. The thin-film circuit layer 102 includes an element region 103 andthe low-strength region 104 surrounding the element region 103. Theelement region 103 includes the transistors. In the method, as shown inFIG. 7E, the grooves can be formed in the low-strength region 104 in thestep of forming the contact holes extending to the source and drainelectrodes 206.

Fourth Embodiment

A method for manufacturing a thin-film circuit device according to afourth embodiment of the present invention will now be described withreference to FIGS. 8A to 8G. In the method, a gate electrode materialfor forming planar thin-film transistors is used as an etching stopperin a step of forming grooves having a rectangular shape in cross section

As shown in FIG. 8A, a protective layer 201 is formed on a substrate 101by a CVD process or another process using silicon dioxide or anothercompound. A silicon layer is formed on the protective layer 201 by a CVDprocess or another process. The silicon layer is subjected to lasercrystallization and then heat treatment, whereby the silicon layer isconverted into a polysilicon layer 202.

As shown in FIG. 8B, the polysilicon layer 202 is patterned, whereby atransistor-forming region is formed. Silicon dioxide is deposited on thepolysilicon layer 202 by a CVD process, whereby a gate insulating layer203 is formed.

As shown in FIG. 8C, a layer of a conductive material such as aluminumis deposited on the gate insulating layer 203 by a sputtering process oranother process and then patterned, whereby a gate electrode-wiringlayer 204 and an etching stopper layer 204 a are formed. The etchingstopper layer 204 a is located in a low-strength region 104. Impuritiesare implanted into the transistor-forming region present in thepolysilicon layer 202 using the gate electrode-wiring layer 204 as amask. The impurities are then activated by heat treatment or the like,whereby source regions and drain regions are formed.

As shown in FIG. 8D, silicon dioxide is deposited over the gateinsulating layer 203 and the gate electrode-wiring layer 204 by a CVDprocess, whereby an interlayer insulating layer 205 is formed.

As shown in FIG. 8E, the interlayer insulating layer 205 and the gateinsulating layer 203 are patterned by an etching process, wherebycontact holes are formed in the transistor-forming region so as toextend to the respective source and drain regions. In this step, thegrooves are formed in a low-strength region 104 in such a manner thatthe interlayer insulating layer 205 is patterned using the etchingstopper layer 204 a as a mask. An etching solution used contains, forexample, hydrofluoric acid such that the etching solution has a highetching rate with respect to the interlayer insulating layer 205 and alow etching rate with respect to the etching stopper layer 204 a.

As shown in FIG. 8F, a layer of a conductive material such as aluminumis deposited on the interlayer insulating layer 205 by a sputteringprocess and then patterned, whereby source and drain electrodes 206 areformed.

As shown in FIG. 8G, silicon nitride or silicon dioxide is depositedover the interlayer insulating layer 205 and the source and drainelectrodes 206, whereby a protective layer 207 is formed. Contact holesare formed in the protective layer 207. A layer of a conductive materialis deposited on the protective layer 207 and then patterned, whereby awiring layer 208 connected to transistor electrodes is formed.

This provides a thin-film circuit layer 102 disposed on the substrate101. The thin-film circuit layer 102 includes an element region 103 andthe low-strength region 104 surrounding the element region 103. Theelement region 103 includes the transistors. In the method, as shown inFIG. 8E, the grooves can be formed in the low-strength region 104 in thestep of forming the contact holes extending to the source and drainelectrodes 206.

Fifth Embodiment

A fifth embodiment of the present invention provides a method formanufacturing a thin-film circuit device including inverted staggeredthin-film transistors. In the method, gate electrodes included in theinverted staggered thin-film transistors are used as etching stoppers ina step of forming grooves having a rectangular shape in cross section.

Sixth Embodiment

A method for manufacturing a thin-film circuit device according to asixth embodiment of the present invention will now be described withreference to FIGS. 9A to 9G. In the method, a base layer for formingplanar thin-film transistors is used as an etching stopper in a step offorming grooves having a rectangular shape in cross section. Althoughthe method is described using a procedure for fabricating the planarthin-film transistors, the method can be used regardless of the type ofthin-film transistors.

As shown in FIG. 9A, a protective layer 201 is formed on a substrate 101by a CVD process or another process using silicon nitride or anothercompound. A silicon layer is formed on the protective layer 201 by a CVDprocess or another process. The silicon layer is subjected to lasercrystallization and then heat treatment, whereby the silicon layer isconverted into a polysilicon layer 202.

As shown in FIG. 9B, the polysilicon layer 202 is patterned, whereby atransistor-forming region is formed. Silicon dioxide is deposited on thepolysilicon layer 202 by a CVD process, whereby a gate insulating layer203 is formed.

As shown in FIG. 9C, a layer of a conductive material such as aluminumis deposited on the gate insulating layer 203 by a sputtering process oranother process and then patterned, whereby a gate electrode-wiringlayer 204 is formed. Impurities are implanted into thetransistor-forming region using the gate electrode-wiring layer 204 as amask. The impurities are then activated by heat treatment or the like,whereby source regions and drain regions are formed.

As shown in FIG. 9D, silicon dioxide is deposited over the gateinsulating layer 203 and the gate electrode-wiring layer 204 by a CVDprocess, whereby an interlayer insulating layer 205 is formed.

As shown in FIG. 9E, the interlayer insulating layer 205 and the gateinsulating layer 203 are patterned by an etching process, wherebycontact holes are formed in the transistor-forming region so as toextend to the respective source and drain regions. In this step, thegrooves are formed in a low-strength region 104. An etching solutionused contains, for example, HF and H₂O so as to have a large etchingrate with respect to silicon dioxide and a small etching rate withrespect to silicon and silicon nitride. The protective layer 201, whichis made of silicon nitride, serves as an etching stopper.

As shown in FIG. 9F, a layer of a conductive material such as aluminumis deposited on the interlayer insulating layer 205 by a sputteringprocess and then patterned, whereby source and drain electrodes 206 areformed.

As shown in FIG. 9G, silicon nitride or silicon dioxide is depositedover the interlayer insulating layer 205 and the source and drainelectrodes 206, whereby a protective layer 207 is formed. Contact holesare formed in the protective layer 207. A layer of a conductive materialis deposited on the protective layer 207 and then patterned, whereby awiring layer 208 connected to transistor electrodes is formed.

This provides a thin-film circuit layer 102 disposed on the substrate101. The thin-film circuit layer 102 includes an element region 103 andthe low-strength region 104 surrounding the element region 103. Theelement region 103 includes the transistors. In the method, as shown inFIG. 9E, the grooves can be formed in the low-strength region 104 in thestep of forming the contact holes extending to the source and drainelectrodes 206.

Seventh Embodiment

A method for manufacturing a thin-film circuit device according to aseventh embodiment of the present invention will now be described withreference to FIGS. 10A to 10E. In the method, grooves having arectangular shape in cross section are formed by processing aninterlayer insulating layer for forming thin-film transistors. Althoughthe method is described using a procedure for fabricating planarthin-film transistors, the method can be used regardless of the type ofthin-film transistors.

As shown in FIG. 10A, an interlayer insulating layer 205 is formed bythe same procedure as that described with reference to FIGS. 9A to 9D.

As shown in FIG. 10B, the interlayer insulating layer 205 and a gateinsulating layer 203 are patterned, whereby contact holes are formed ina transistor-forming region so as to extend to source and drain regions.

As shown in FIG. 10C, a layer of a conductive material such as aluminumis deposited on the interlayer insulating layer 205 by a sputteringprocess and then patterned, whereby source and drain electrodes 206 areformed.

As shown in FIG. 10D, silicon nitride or silicon dioxide is depositedover the interlayer insulating layer 205 and the source and drainelectrodes 206, whereby a protective layer 207 is formed. The protectivelayer 207 is patterned, whereby contact holes are formed on the sourceregions and the drain regions and the grooves are formed in alow-strength region 104 located in the protective layer 207.

As shown in FIG. 10E, a layer of a conductive material is deposited onthe protective layer 207 and then patterned, whereby a wiring layer 208connected to transistor electrodes is formed.

This provides a thin-film circuit layer 102 disposed on the substrate101. The thin-film circuit layer 102 includes an element region 103 andthe low-strength region 104 surrounding the element region 103. Theelement region 103 includes the transistors. In the method, as shown inFIG. 10D, the grooves can be formed in the low-strength region 104 inthe step of forming the contact holes extending to the source and drainelectrodes 206.

Eighth Embodiment

A method for manufacturing a thin-film circuit device according to aneighth embodiment of the present invention will now be described withreference to FIGS. 11A and 11B. In this embodiment, a step of forminggrooves having a V shape in cross section is described in detail.Although the method is described using a procedure for fabricatingplanar thin-film transistors, the method can be used regardless of thetype of thin-film transistors.

FIG. 11A shows a thin-film circuit substrate including thin-filmtransistors fabricated by a procedure for fabricating the planarthin-film transistors described above (for example, a procedureincluding steps which are similar to those shown in FIGS. 10A to 10E andin which no low-strength region is formed). A photoresist 210 is appliedto a protective layer 207 disposed above the substrate by a spin-coatingprocess or another process. A photomask, which is not shown, having apattern corresponding to the grooves is placed on the photoresist 210.The photoresist 210 is lightly exposed and then developed. This allowsthe photoresist 210 to have a plurality of V-shaped groovescorresponding to those grooves. The protective layer 207 isanisotropically etched using the photoresist 210 as a mask, whereby theV-shaped grooves are transferred to the protective layer 207. Thephotoresist 210 is removed and the substrate is then cleaned.

This provides the thin-film circuit device having those grooves as shownin FIG. 11B. Those grooves extend in a portion of the protective layer207 that corresponds to a low-strength region 104.

Ninth Embodiment

A method for manufacturing a thin-film circuit device according to aninth embodiment of the present invention will now be described withreference to FIGS. 12A and 12B. In this embodiment, a step of forminggrooves which have a semi-elliptical or semi-circular shape in crosssection or which partly have such a shape in cross section is describedin detail. Although the method is described using a procedure forfabricating planar thin-film transistors, the method can be usedregardless of the type of thin-film transistors.

FIG. 12A shows a thin-film circuit substrate including thin-filmtransistors fabricated by a procedure for fabricating the planarthin-film transistors described above (for example, a procedureincluding steps which are similar to those shown in FIGS. 10A to 10E andin which no low-strength region is formed). A photoresist 210 is appliedto a protective layer 207 disposed above the substrate by a spin-coatingprocess or another process. A photomask, which is not shown, having apattern corresponding to the grooves is placed on the photoresist 210.The photoresist 210 is strongly exposed and then developed. This allowsthe photoresist 210 to have a plurality of inverted trapezoid-shapedgrooves corresponding to those grooves. The protective layer 207 iswet-etched with an etching solution using the photoresist 210 as a mask,whereby the inverted trapezoid-shaped grooves are transferred to theprotective layer 207. The grooves transferred to the protective layer207 have a semi-elliptical or semi-circular shape depending onproperties of the etching solution. The photoresist 210 is removed andthe substrate is then cleaned.

This provides the thin-film circuit device having those grooves as shownin FIG. 12B. Those grooves extend in a portion of the protective layer207 that corresponds to a low-strength region 104.

As described above, the low-strength region 104 can be formed in athin-film circuit layer 102 including thin-film transistors.

Tenth Embodiment

A method for manufacturing thin-film circuit devices 100 according to atenth embodiment of the present invention will now be described withreference to FIGS. 13A to 13D and 14A to 14D. The thin-film circuitdevices 100 are flexible and are manufactured in such a manner thatthin-film circuit layers 102 including low-strength regions 104 areformed above a substrate 101 made of glass and the thin-film circuitlayers 102 are transferred to a flexible resin film.

The thin-film circuit devices 100 are similar to the thin-film circuitdevice 100 of Seventh Embodiment described with reference to FIGS. 10Ato 10E. The thin-film circuit device according to any one of the otherembodiments may be used to describe the method of this embodiment.

As shown in FIG. 13A, an amorphous silicon layer 101 a serving as arelease layer is formed on the substrate 101 by a CVD process. Thethin-film circuit layers 102 are formed on the amorphous silicon layer101 a by the same procedure as that described in Seventh Embodiment.

As shown in FIG. 13B, the thin-film circuit layers 102 are joined to arelease layer 302, made of amorphous silicon, lying on a temporarytransfer substrate 301 with a water-soluble adhesive 303.

As shown in FIG. 13C, the lower face of the substrate 101 is irradiatedwith laser beams, whereby the bonds between silicon atoms in theamorphous silicon layer 101 a are broken.

As shown in FIG. 13D, the substrate 101 is separated from the thin-filmcircuit layers 102. The thin-film circuit layers 102 remain under thetemporary transfer substrate 301.

As shown in FIG. 14A, the thin-film circuit layers 102 are joined to aflexible resin substrate 401 with a water-insoluble adhesive 402.

As shown in FIG. 14B, the upper face of the temporary transfer substrate301 is irradiated with laser beams, whereby the bonds between siliconatoms in the release layer 302 are broken.

As shown in FIG. 14C, the temporary transfer substrate 301 is separatedfrom the thin-film circuit layer 102. The thin-film circuit layers 102remain on the resin substrate 401. The water-soluble adhesive 303 isremoved by water washing.

As shown in FIG. 14D, the thin-film circuit devices 100 are obtained.The thin-film circuit devices 100 include the thin-film circuit layers102 including element regions 103 and low-strength regions 104surrounding the respective element regions 103. The thin-film circuitdevices 100, as well as those shown in FIG. 2, are arranged above theresin substrate 401.

The above method is called a release transfer process, which is usefulin manufacturing thin-film circuit devices at high temperature. Thesethin-film circuit devices include transistors having higher performancethan that of transistors that are directly fabricated above a flexiblesubstrate having low heat resistance at low temperature.

Eleventh Embodiment

An electronic apparatus according to an eleventh embodiment of thepresent invention will now be described. The electronic apparatusincludes the thin-film circuit device according to any one of the aboveembodiments.

FIGS. 15A to 15C are illustrations of examples of the electronicapparatus. FIG. 15A shows a mobile phone 1000 including a displaysection 1001. The display section 1001 includes an electro-optical unitincluding the thin-film circuit device. The electro-optical unit furtherincludes a liquid crystal display panel, an organic electroluminescent(EL) panel, an electrophoretic display panel, or another panel.

FIG. 15B shows a video camera 1100 including a display section 1101. Thedisplay section 1101 includes an electro-optical unit including thethin-film circuit device.

FIG. 15C shows a television 1200 including a display section 1201. Thedisplay section 1201 includes an electro-optical unit including thethin-film circuit device. An electro-optical unit including thethin-film circuit device can be used for a monitor for personalcomputers.

Examples of the electronic apparatus include facsimile machines, digitalcameras, portable televisions, personal digital assistants (PDAs),electronic notebooks, electronic billboards, and advertising displays.

As described above, a thin-film circuit device according to a preferredembodiment of the present invention is advantageous in that alow-strength region can prevent cracks developed in an end portion ofthe thin-film circuit device from propagating to an element region. Thatis, the cracks reach the low-strength region and then propagate in thelow-strength region in the direction in which the low-strength regionextends. Since the element region is surrounded by the low-strengthregion, the cracks are prevented from propagating to the element region.This protects the element region.

1. A thin-film circuit device comprising: a substrate; and a thin-filmcircuit layer, disposed on the substrate, having an element region and alow-strength region, wherein the element region includes thin-filmelements and the low-strength region extends between an end portion ofthe thin-film circuit layer and the element region and has a mechanicalstrength less than that of the surroundings of the low-strength region.2. A thin-film circuit device comprising: a substrate; and a thin-filmcircuit layer, disposed on the substrate, having an element region and alow-strength region, wherein the element region includes thin-filmelements and the low-strength region extends between an end portion ofthe thin-film circuit layer and the element region to surround theelement region and has a mechanical strength less than that of thesurroundings of the low-strength region.
 3. The thin-film circuit deviceaccording to claim 1, wherein the low-strength region surrounds theelement region several times.
 4. The thin-film circuit device accordingto claim 1, wherein the low-strength region has a groove surrounding theelement region.
 5. The thin-film circuit device according to claim 1,wherein the low-strength region has a plurality of grooves surroundingthe element region.
 6. The thin-film circuit device according to claim4, wherein the groove or grooves have a V shape, an inverted trapezoidalshape, a rectangular shape, a semi-circular shape, or a semi-ellipticalshape in cross section or partly have any one of these shapes in crosssection.
 7. An electronic apparatus comprising the thin-film circuitdevice according to claim
 1. 8. A method for manufacturing a thin-filmcircuit device including a substrate and a thin-film circuit layerdisposed on the substrate, comprising: forming an element regionincluding thin-film elements on the substrate; and forming alow-strength region having a low mechanical strength around the elementregion, wherein the low-strength region is formed simultaneously withthe formation of contact holes in the element region.
 9. The methodaccording to claim 8, wherein the low-strength region is formed byetching the thin-film circuit layer.
 10. The method according to claim8, further comprising fabricating the thin-film circuit layer on aheat-resistant substrate, releasing the thin-film circuit layer from theheat-resistant substrate, and then transferring the thin-film circuitlayer to a flexible substrate.